1. Field of the Invention
The present invention relates to a radio communication device and, in particular, radio communication devices for use in a multi-channel radio communication system.
2. Discussion of the Related Art
In the field of radio communications, multiple-input multiple-output (MIMO) systems transmit signals over at least two antennas and receive signals over at least two antennas. A typical MIMO communication system is shown in FIG. 1, which is described in more detail below. A data stream is split into a number of independent sub-streams and each sub-stream is transmitted over a different antenna using the same frequency band. Each signal is coded to be statistically independent from the other signals. At the receiver, each antenna receives the signals transmitted by all of the transmit antennas, as well as any multipath signals arising in the local environment. The received signals are processed to isolate individual data sub-streams, to the maximum extent possible according to channel conditions, which are then recombined to recreate the original datastream.
The separate transmit signals may be coded to be statistically independent using orthogonal frequency division multiple access (OFDMA) techniques or code division multiple access (CDMA) techniques. Coding ensures low correlation between signals.
The throughput of a MIMO system promises to be much greater than an equivalent SISO (single-input single-output) system. This additional throughput can potentially come with no increase in bandwidth or overall transmit power. MIMO systems work best in environments in which there is a large amount of scattering, such as environments which can be modelled with independent Rayleigh fading. Scattering of this nature functions as a spatial multiplexer, which spatially decorrelates the signals. In such environments, the receive antennas receive independent linear combinations of the signals transmitted along many paths. It follows that outputs of the receive antennas are linear combinations of the transmitted signals which can distinguished from each other.
In a conventional MIMO system, coupling between receive antennas occurs according to two mechanisms. Firstly, each receive antenna receives signals from the transmitter which induce currents in the conductive structure of the antenna. The induced currents generate their own electromagnetic fields which re-radiate corresponding signals. These re-radiated signals are received by adjacent antennas. Secondly, in a typical system, signals generated by the antennas result in currents in a receiver ground plane and these currents are shared by adjacent antennas. For example, with a monopole quarterwave (λ/4) antenna over a ground plane, currents in the ground plane mimic an equivalent quarterwave dipole below the ground plane. By whichever mechanism coupling occurs between receive antennas, it is a serious problem as it causes an increase in the correlation between the signals fed from the receive antennas to receiver circuitry coupled to the antennas. These effects are particularly severe when the receive antennas are close together.